RIG-I like receptors (RLRs) detect viral infections and triggers appropriate immune responses to limit viral infection and spread. RIG-I, the best studied member of RLRs, binds to viral RNA in the cytosol and triggers a signaling cascade that leads to the production of type-I interferons and other antiviral molecules. We have previously identified a mitochondrial antiviral signaling protein termed MAVS (also known as IPS-1, VISA or CARDIF), which plays a pivotal role in the RIG-I signaling cascade. MAVS resides on the mitochondrial outer membrane and it activates cytosolic protein kinases including IKK and TBK1, which phosphorylate the transcription factors NF-?B and IRF3, respectively. NF-?B, IRF3 and other transcription factors function together in the nucleus to turn on the expression of antiviral genes. In order to understand the mechanism of signal transduction in the RIG-I pathway, we have recently established a cell-free system that faithfully recapitulates the activation of IRF3 by viral RNA. Initial dissection of this system leads to the discovery that RIG-I activation requires not only viral RNA, but also a unique form of polyubiquitination involving lysine-63 (K63) of ubiquitin. Further investigation shows that unanchored K63 polyubiquitin chains, which are not conjugated to any cellular protein, binds to the N-terminal CARD domains of RIG-I, resulting in RIG-I activation. These studies provide a strong foundation for further dissection of the RIG-I signaling pathway. In this application, we propose to elucidate the mechanism of RIG-I regulation by RNA and K63 ubiquitin chains (Aim 1). We will also investigate how RIG-I activates MAVS on the mitochondrial membrane (Aim 2). Finally, we will delineate the pathways through which MAVS activate TBK1 and IKK in the cytosol (Aim 3). Together, these lines of investigation should fill some of the major gaps in our current understanding of the RIG-I signaling cascade, and provide mechanistic insights into antiviral innate immunity.